As information processing and information communication technology has been developed in recent years, computers have been gaining widespread use and computers have been actively used even in daily life. For example, the use of a mobile communication terminal such as a smartphone or a mobile phone terminal makes it possible to view information on a network or enjoy a service of navigation to a destination when going out.
In addition, as a service that converts various kinds of text information on networks into speech and allows it to be enjoyed, for example, Patent Literature 1 below proposes a sound synthesizing method that selects articles having high interest degrees one by one from an article group in descending order of interest degrees with a condition set such as total audio time length, speech speed, or the number of essential reproduction articles, summarizes a selected article to an approximate amount, and reproduces the article with synthesized sound.